Set foreign key values based on existing column data - mysql

Question

I am in the middle of migrating an old (unnormalized) database to its new version.
Right now I have this intermediate result:
CREATE TABLE recipient(
id int NOT NULL AUTO_INCREMENT,
email VARCHAR(255),
PRIMARY KEY (id),
UNIQUE INDEX (`email`),
) ENGINE=INNODB;
CREATE TABLE comment(
# Right now this is always NULL:
fk_recipient INT,
# Temporary solution. Note that this field is NOT UNIQUE:
tmp_email VARCHAR(255) NOT NULL,
# ...
FOREIGN KEY (`fk_recipient`) REFERENCES recipient(id);
) ENGINE=INNODB;
Both tables are filled with correct data:
some million comments with the right tmp_email and fk_recipient = null in table comment (note: emails are not unique)
some hundred thousand UNIQUE email adresses in table recipient.
What I need to do:
I want to get rid of the comment.tmp_email column and instead point comment.fk_recipient to the appropriate row in table recipient.
My current approach (using PHP):
get all comments
iterate over all comments:
look up the right row in recipient table
set right foreign key
... DROP COLUMN tmp_email
This takes forever and made me wonder if there is no native MySQL way to do that?

Answer 1

The following workaround will do the job:
Create temporary foreign key on comment.tmp_email:
ALTER TABLE comment
ADD CONSTRAINT `tmpMailKey` FOREIGN KEY (`tmp_email`)
REFERENCES `recipient`(`email`);
Join the two tables on the temporary key and use the information to set the real foreign key:
UPDATE comment c
INNER JOIN recipient r
ON
c.tmp_email = r.email
AND c.tmp_email IS NOT NULL
SET c.fk_recipient = r.id;
Get rid of temporary foreign key (and the tmp column too):
ALTER TABLE `udw_v3`.`travelogue_guestbookentry`
DROP COLUMN `tmp_email`,
DROP INDEX `tmpMailKey`,
DROP FOREIGN KEY `tmpMailKey`;